Drugs of abuse effect human aggressive and conversational behavior. However, the effects of these drugs on human cooperative responding remains to be examined. This proposal describes a free-operant laboratory procedure for examining the effects of caffeine, diazepam, ethanol and nicotine in human cooperative responding. In the proposed studies cooperative behavior will be established and maintained by experimental contingencies that schedule two periods. During one period subjects working alone can earn points exchangeable for money by pressing a button. During the other period, ostensibly initiated by another person paired with the subject, subjects can either work with the other person or work independently. Working with the other person, the cooperative responses, will be maintained by points exchangeable for money added simultaneously to the subject's and the fictitious partner's counter. Both counters will be visible to the subject. Working independently, the non-cooperative option, will be maintained by point exchangeable for money being added only to the subject's counter. Experiments will also examine drug effects on cooperative exchanges initiated by the subject. These experiments extend the study of drug effects on human social behavior to cooperative behavior, a behavior vital to the functioning of human social and work groups.